internal/pprof/profile: parse mutex profile including comments

Skip lines if they are empty or starting with "#" which are valid
legacy pprof output format.

Fixes #18025

Change-Id: I7aee439171496932637b8ae3188700911f569b16
Reviewed-on: https://go-review.googlesource.com/33454Reviewed-by: Peter Weinberger <pjw@google.com>

src/internal/pprof/profile/legacy_profile.go

@@ -686,10 +686,19 @@ func scaleHeapSample(count, size, rate int64) (int64, int64) {
 // the runtime might write a serialized Profile directly making this unnecessary.)
 func parseContention(b []byte) (*Profile, error) {
 	r := bytes.NewBuffer(b)
-	l, err := r.ReadString('\n')
+	var l string
+	var err error
+	for {
+		// Skip past comments and empty lines seeking a real header.
+		l, err = r.ReadString('\n')
 		if err != nil {
-		return nil, errUnrecognized
+			return nil, err
 		}
+		if !isSpaceOrComment(l) {
+			break
+		}
+	}
+
 	if strings.HasPrefix(l, "--- contentionz ") {
 		return parseCppContention(r)
 	} else if strings.HasPrefix(l, "--- mutex:") {
@@ -729,6 +738,9 @@ func parseCppContention(r *bytes.Buffer) (*Profile, error) {
 				break
 			}
 		}
+		if isSpaceOrComment(l) {
+			continue
+		}
 
 		if l = strings.TrimSpace(l); l == "" {
 			continue
@@ -773,6 +785,7 @@ func parseCppContention(r *bytes.Buffer) (*Profile, error) {
 
 	locs := make(map[uint64]*Location)
 	for {
+		if !isSpaceOrComment(l) {
 			if l = strings.TrimSpace(l); strings.HasPrefix(l, "---") {
 				break
 			}
@@ -799,6 +812,7 @@ func parseCppContention(r *bytes.Buffer) (*Profile, error) {
 				Value:    value,
 				Location: sloc,
 			})
+		}
 
 		if l, err = r.ReadString('\n'); err != nil {
 			if err != io.EOF {

src/internal/pprof/profile/profile_test.go

@@ -22,3 +22,58 @@ func TestEmptyProfile(t *testing.T) {
 		t.Errorf("Profile should be empty, got %#v", p)
 	}
 }
+
+func TestParseContention(t *testing.T) {
+	tests := []struct {
+		name    string
+		in      string
+		wantErr bool
+	}{
+		{
+			name: "valid",
+			in: `--- mutex:
+cycles/second=3491920901
+sampling period=1
+43227965305 1659640 @ 0x45e851 0x45f764 0x4a2be1 0x44ea31
+34035731690 15760 @ 0x45e851 0x45f764 0x4a2b17 0x44ea31
+`,
+		},
+		{
+			name: "valid with comment",
+			in: `--- mutex:
+cycles/second=3491920901
+sampling period=1
+43227965305 1659640 @ 0x45e851 0x45f764 0x4a2be1 0x44ea31
+#	0x45e850	sync.(*Mutex).Unlock+0x80	/go/src/sync/mutex.go:126
+#	0x45f763	sync.(*RWMutex).Unlock+0x83	/go/src/sync/rwmutex.go:125
+#	0x4a2be0	main.main.func3+0x70		/go/src/internal/pprof/profile/a_binary.go:58
+
+34035731690 15760 @ 0x45e851 0x45f764 0x4a2b17 0x44ea31
+#	0x45e850	sync.(*Mutex).Unlock+0x80	/go/src/sync/mutex.go:126
+#	0x45f763	sync.(*RWMutex).Unlock+0x83	/go/src/sync/rwmutex.go:125
+#	0x4a2b16	main.main.func2+0xd6		/go/src/internal/pprof/profile/a_binary.go:48
+`,
+		},
+		{
+			name:    "empty",
+			in:      `--- mutex:`,
+			wantErr: true,
+		},
+		{
+			name: "invalid header",
+			in: `--- channel:
+43227965305 1659640 @ 0x45e851 0x45f764 0x4a2be1 0x44ea31`,
+			wantErr: true,
+		},
+	}
+	for _, tc := range tests {
+		_, err := parseContention([]byte(tc.in))
+		if tc.wantErr && err == nil {
+			t.Errorf("parseContention(%q) succeeded unexpectedly", tc.name)
+		}
+		if !tc.wantErr && err != nil {
+			t.Errorf("parseContention(%q) failed unexpectedly: %v", tc.name, err)
+		}
+	}
+
+}